Electrolysis of alkali-metal chlorides



Patented June 30, 1925 AT l CHARLES E. VAUGHN AND RALPH E. GEGE-NI-IEIMER, OF NIAGARA FALLS, NEW YORK, I

ASSIGNORS TO THE MATHIESON ALKALI "WORKS, INC., OF NEW YORK, N. Y., A

CORPORATION OF VIRGINIA.

ELECTROLYSIS OF ALKALI-METAL CHLORIDES.

1N0 Drawing.

To all whom it may concern Be it known that We, CHARLES F. VAUGHN and RALPH E. GEGENHEIMER, both citizens of the United States, and both residing at Niagara Falls, in the county of Niagara, State of New York, have invented certain new and useful Improvements in Electrolysis of Alkali-Metal Chlorides; and We do hereby declare the following to be a full, clear, and exact description of the invention, such as will enable others skilled in the art to which it appertains to make'and use the same.

This invention relates to improvements in electrolytic processes carried out with a mercury cathode, in the course of Which an amalgam of an alkali metal is formed; and more particularly to improvements in the electrolysis of solutions of sodium or potassium chloride in cells having a mercury cathode.

In the electrolysis of solutions of alkali metal chlorides in mercury cathode cells, for example, according to the Castner process for making potassium or sodium hydroxide, a solution made from commercial salt or muriate is continuously circulated and only a small portion of the dissolved salt is decomposed on each passage through the cells.

.WlllCh are arranged in parallel. The brine or solution of alkali metal chloride returning from the cells is commonly passed through a trap Where the greater part of the entrained chlorine is removed, but free chlorine is still contained in the solution, e. g., to the extent of about 0.5 gram per liter. The solution is then passed through a bed of salt or muriate in a saturator and is resaturated With the alkali metal chloride, and the resaturated solution or brine is then returned through the cells.

If raw salt (sodium chloride) is used in the saturator for the resaturation of the chlorinated brine, the resaturated brine may contain impurities, which are thus introduced into the circulation either in suspen sion or solution. In order to remove insoluble impurities, the brine flows through settling chambers Where the velocity is sufliciently reduced to permit of the settling out of the larger particles of impurities before the brine is returned through the cells.

In the absence of any soluble impurities in the salt used for res-aturation, and in the Application filed January 7, 1921. Serial no. 435,738.

resaturated solution or brine, other than the variably present in raw salt, the electrolytic process Will operate satisfactorily in mercury cathode cells according to the procedure above described; and it is one of the advantages of such mercury cathode cell processes that they permit of the use of raw salt Without purification from calcium and magnesium compounds; whereas, in diaphragm cell processes of electrolysis, calcium and magnesium compounds precipitate and clog the diaphragm, so that they must be removed before the brine can be used.

\Vhen, however, the raw salt used in re saturating the brine, in the mercury cathode cell process, contains impurities, such as iron compounds, that are attacked by the chlorine, these impurities will be introduced in solution into the circulation. When iron thus dissolves in the chlorinated brine and is carried into the electrolytic cells, the process is very seriously affected, as even small amounts of iron in solution cause decomposition of the mercury amalgam and greatly .loWer the efficiency of the cell, besides causing dangerous percentages of hydrogen to come otf With the chlorine gas. This action of iron compounds is greatly increased by the presence of the calcium and magnesium salts which are always found in unpurified salt solutions.

The present invention is based upon the discovery that the objectionable decomposition and decreased efficie 03 due to iron compounds, which may b dissolved in the chlorinated brine during resaturation and thereby introduced into the circulation, can be avoided by so carrying out the process that the iron will be precipitated by the brine constituents normally present in small amount in an electrolyte using a mercury cathode, and by separating the precipitated impurities, by filtration, before the brine isv returned to the electrolytic cells. We have found it of importance not only to bring about the precipitation of the iron before the brine returns to the,- cells, but also the removal of the precipitated iron, as by filtration, before the return of the brineto the cells, since the precipitate forms gradually and over an appreciable period of time'and is to a greater or less extent in such a finely divided state that it does not settle out 5R calcium and magnesium compounds, in-' saturator, have been precipitated, by reaction with the brine constituents, in an insoluble form permitting their removal.

In using raw salt which contained iron impurities, for the resaturation of the chlorinated brine, we have found that the resulting brine contained iron in varying amounts up to about .010 gram or more per liter. With brine also containing an average of 0.5 gram of calcium and .01 gram of magnesium per liter, even such traces of iron were suflicient to cause a very serious inetiiciency in the electrolytic cell, and the formation of hydrogen in objectionable amount in the anode chamber. The iron which is thus dissolved by the action of the chlorine during resaturation can be precipitated more or less completely in the form of a finely divided precipitate, but this precipitate takes an appreciable time for its formation and does not readily settle out; and, if the brine is returned to the cells before the precipitation is complete, or if the brine is returned with the precipitate carried in suspension therein, the ironin solution or in suspension will bring about decreased efficiency of operation, the precipitate being active in much the same manner as iron salts in solution.

Accordingly, in carrying out the process of the present invention, we provide not only for the substantially complete precipitation of the iron by suitable regulation of the process, but also for the removal of the precipitated iron by filtration, so that the brine will return to the electrolytic cells substantially free from both iron in solution and precipitated iron.

The precipitation of the iron from the brine can be made to take place with substantial completion under proper conditions of operation, e. g., at a temperature of about 60 Q, and if a sufiicient period of time is permitted, for example, approximately one hour from the time the brine leaves the saturator until it has passed through the settlers and the filter bed. While we do not Wish to limit ourselves by any theoretical explanatlon of the exact cause of this precipitation of the iron, yet we believe (and our investigations seem to confirm this view) that it is due to the action of sodium hypochlorite which is present in small amount in the brine; and the resulting active precipitate is probably a hydrated iron oxide, although its composition has not been definitely established. We have found that the concentration of sodium hypochlorite in the chlorinated brine is variable, but .in all cases low, and it is for this reason, as we believe, that an appreciable time is required to precipitate all of the iron.

The precipitate formed is so finely divided that it does not readily settle out in the settlers, but we have found that it can readily be removed, after sufficient opportunity has been provided for its formation, by the simple expedient of filtering the chlorinated brine after it has passed the settlers and before it returns to the cell.

The present invention, accordingly, involves the regulation of the brine circulation so that sufiicient time is permitted for the iron to be precipitated in a removable form, by reaction with constituents normally present in the brine, and the subsequent separation of the precipitated iron,

before the brine re-enters the cells for further electrolysis.

lVe have a-lso'found that some of the iron may precipitate in the chlorinated brine in an inactive form, that is, in a form which does not effect the mercury amalgam;-but, even where some such inactive precipitate is formed, the iron is precipitated to a considerable extent at least in the form of an ac tive precipitate which is objectionable if introduced into the electrolytic cell. According to the present invention, such active precipitate is removed, so that its objectionable action upon the amalgam and upon the electrolytic operation is avoided.

In the practice of the invention, the 0p erati-on of the mercury cathode cells can be carried out in the usual way, as in the usual operation of the Castner process, and the temperature of the brine maintained within proper limits, for example, between 40 and C. Instead, however, of using salt which is free from iron for resaturating the brine during its circulation, raw salt is used which may contain iron in varying amount; and the objectionable effect of the iron taken into solution by the chlorinated brine will nevertheless be avoided. The resaturation of the brine can thus be effected in much the usual way by passing it up through a bed of raw salt fed into the saturator, so that the brine will be saturated, while in contact therewith. The brine then flows through settling chambers where the velocity is sufficiently reduced topermit of the settling out of the larger particles of impurities, such as shale and undissolved gypsum; and the rate and time of flow of the brine is so regulated that a sufficient opportunity is provided for the precipitation of the iron in an insoluble form. The brine is then passed through a filter, which may be an ordinary sand filter of sufficient depth, and the resulting purified brine is then returned to' the electrolytic cell. By carrying out the operation in this way, removal of iron compounds is effected and good results can be obtained; whereas, if the iron is not removed but is carried into the cell, either in solution or in the active form of precipitate, the operation of the Castner piocess is very seriously attected, with objectionable decomposition of the mercury amalgam in the anode chamber and greatly lowered efficiency of the cell, and with dangerous percentages of hydrogen coming off with the chlorine gas.

I t will thus be seen that the present invention involves an important improvement in the operation of electrolytic cells with a mercury cathode, when the salt used for resaturation of the circulated brine contains iron or other similarly active impurities. According to the present invention, these impurities in the brine, which cause the decomposition of the amalgam during the electrolysis and decreased efficiency, are removed and their objectionable efi'ects avoided. So also, the purification of the brine or salt solution, according to the present invention, may be considered a self-purification, inasmuch as the precipitation of the iron is brought about by constituents normally present in the brine in small and varying amount, by aifording a sufficient time and opportunity for this self-purification and precipitation to take place, and by filtering off the precipitate so formed.

It is one advantage of the present invention that it does not involve any radical change in the operation of the electrolytic process and the circulation and resaturation of the chlorinated brine, but merely requires that the circulation be so regulated that the impurities will be precipitated in an insoluble form, and the subsequent removal of these impurities from suspension in the brine, which removal can be effected by filtration. The brine can thus be returned to the cells in a saturated state, freed from the objectionable impurities, and still at a suitable temperature of e. g., 40 to C.

While we have described the process of the invention more particularly in connection with the electrolysis of a sodium chloride brine, for the production of chlorine and caustic soda, yet the process can be carried out in a similar manner with solutions of other alkali metal salts, such as solutions of potassium chloride, where the salt for resaturating the brine contains impurities which are taken into solution during the resaturation, and which can be removed from the saturated solution in the manner above described, i. e., by afiording sufficient time and opportunity for precipitation of the impurities, and by filtering off the precipitated impurities.

lVhile the process of the --present invention can be carried out to advantage without the addition of any added reagent for bringing about precipitation, and by taking advantage of the small and varying amounts of precipitating agent or agents normally pres ent as constituents of the brine or salt solu tion, yet we do not exclude the addition of further small amounts of such constituent. or precipitating agents, e. g, small amounts of sodium hypochlorite, to increase the con centration of 'the precipitating constituents and cause the precipitation to take place a a more rapid rate, so that less time will bf required for the completion of 'the precipi tation prior to the filtration and return of the brine. or solution to the electrolytic cells So also, while we have described the in vention in connection with the precipitation of iron as an impurity of the resaturated brine or salt solution, we do not limit our selves to the separation of this particular impurity, as other metal impurities, such as manganese compounds, if present. can be precipitated andremoved in a. similar manner, and with similar advantage to the electrolytic operation.

\Ve claim 1. The improvement in the electrolysis of alkali metal salt solutions in cells having a mercury cathode, which comprises electrolyzing the solution, resaturating the solution with salt containing impurities which are taken into solution and which are ac- .tive during electrolysis, removing such impurities following the resaturation and before returning the solution for electrolysis, and returning the solution for elec trolysis.

2. The improvement in the electrolysls of alkali metal salt solutions in cells having a mercury cathode, which comprises electrolyzing the solution, resaturating the salt solution, purifying the saturated salt solution by precipitating the impurities therefrom by the action of precipitating agents present in the salt solution, separat ng the precipitated impurities, and returning the solution for electrolysis.

3. The improvement in the electrolysls of alkali metal salt solutions in cells having a mercury cathode, which comprises electrolyzing the alkali metal chloride solutlon, resaturating the solution with alkali metal chloride containing iron impurit es, bringing about the precipitation of the 110X1 taken into solution byconstituents normally present in the'solution, removing the impurities, and returning the solution for electrolysis.

4. The improvement in the electrolysis of sodium chloride brine with the use of a mercury cathode, which comprises electrolyzing the brine, resaturating the brine with ;salt containing iron impurities, bringing about precipitation of the iron taken into solution by constituents normally present in the brine, removing the impurities and returning the brine for electrolysis.

5. The improvement in the electrolysis of sodium chloride brine with the use of a mercury cathode, which comprises electrolyzing the brine, resaturating the chlorinated brine With salt containing iron impurities, permitting the coarser impurities to settle from the brine, causing the iron taken into solution during resaturation to be precipiated, filtering the brine to remove the precipitate, and returning the brine for electrolysis.

6. The improvement in the electrolysis of alkali metal chloride solutions with the use of a mercury cathode, which comprises electrolyzing the solution, resaturating the solution with salt containing iron impurities, causing the precipitation of iron impurities by precipitating agents normally contained in the resaturated solution, removing the precipitated iron compounds from the solution by subjecting it to filtration, and returning the solution for electrolysis,

7. The improvement in the electrolysis of alkali metal salt solutions in cells having a mercury cathode and involving the circulation of the solution for electrolysis with subsequent resaturation and returning of the solution for electrolysis, which -comprises so regulating the circulation of the solution that soluble impurities are precipitated, and removing the impurities.

In testimony whereof We affix our signatures.

CHARLES F. VAUGHN. RALPH E. GEGENHEIMER. 

